Surface printing and lamination printing are two examples of printing processes used to print on substrates such as film, or flexible packaging material, which may include foils and metalized films. Printing on those substrates with such processes places certain requirements on the ink composition used. Inks used in surface printing are printed to the front side of the film. The ink must print well, adhere to the substrate, have good water resistance, rub resistance, squalene resistance, and other performance properties consistent with an ink that may experience normal everyday direct handling.
Extrusion lamination and adhesive lamination are two types of lamination printing processes often used with flexible packaging material. Inks used in such processes are generally printed to the reverse side of a substrate. The inked substrate is then laminated to a second barrier substrate with either molten polyethylene extrudate (extrusion lamination) or with a water-based, solvent-based or solventless adhesive (adhesive lamination). The inks must print well and must not detract from the lamination bond strength. The inks must also possess sufficient flexibility, heat, water, solvent and rub resistance to withstand the lamination process and maintain laminate structure after application.
A typical water-based ink, such as a surface printing ink, extrusion lamination ink, and adhesive lamination ink composition contains water dispersible polymers, one or more pigments, water-miscible solvents, and additives such as wetting agents, leveling agents, rheology additives, additives to promote resolubility/rewet on the press, coalescing aids, pigment wetting agents, dispersing agents, surfactants, waxes, defoaming agents and antifoaming agents.
These inks are applied by typical application methods, such as the flexographic and gravure methods common to the ink industry. The inks are printed on multi-color printing presses, thermally dried, and then converted into packaging materials. After printing, a surface printed substrate is re-wound, then shipped to the packaging convertor for production. The extrusion or adhesive lamination process is often done "in-line." The typical "in-line", extrusion lamination method uses molten low density polyethylene (LDPE) as the extrudate. The ink is reverse printed to the first substrate, typically a non rigid film, such as oriented polypropylene (OPP), polyethylene, polyester, polyamide, etc. This printed substrate passes through a drying oven to dry and cure the ink. The ink is then primed with a dilute solution of adhesion promoting polyethyleneiminie or polyurethane. The primed ink is laminated to the second substrate by pressing it in a two roll nip while injecting molten polyethylene at the nip opening. Following lamination, the ink must maintain adhesion to the first substrate, must have cohesive strength and must adhere to the polyethylene extrudate.
An "off-line" extrusion lamination process is similar to the "in-line" process except the substrate is rewound after the ink is cured and dried. The inked substrate is transported to a laminator, unwound, primed with either polyethyleneimine or polyurethane, and then laminated to a second substrate with molten polyethylene in a two roll nip. The ink must maintain adhesion to the first substrate, must maintain cohesive strength, must adhere to the polyethylene extrudate, must not block on the roll, and must have sufficient resistance properties so as not be damaged during transport to the laminator. Typical properties needed to withstand transporting are: water resistance, dry and wet rub resistance and adhesion.
The "in-line" and "off-line" adhesive lamination processes are similar to their respective extrusion processes except the inked substrate is laminated to the second substrate with a solvent-based, water-based, or solventless adhesive. The adhesive is typically applied over the ink immediately prior to lamination. The ink must have sufficient resistance to the adhesive so that ink does not lift from the substrate nor lose properties due to attack by the adhesive. The ink must maintain adhesion to the first substrate, must adhere to the adhesive, must have cohesive strength following lamination, must not lose performance when an adhesive is applied to the ink, and must have sufficient water resistance, non-blocking, and rub resistance properties to allow re-winding and transportation to an laminator for "off-line" lamination.
A surface printing process is also similar to an "off-line" lamination printing process, except that it does not include a laminating step. The substrate is rewound after the ink is cured and dried. The ink must maintain adhesion, not block on the roll, and have sufficient resistance properties so as not to be damaged during further processing into and use as a finished good.